To harden the teeth of wire clothing is a very delicate operation because only the tip of the tooth should be hardened while the tooth base must stay unhardened and the hardness gradient area between hardened and unhardened portions should be kept as reduced as possible. Up to now, this operation is effected with a torch. With this heating mode, the maximal speed is .about.10m/min. The hardness transition gradient area is relatively narrow but it is not possible to further reduce it as desired. Indeed, admittedly, since this hardening process has been used for many decades, the limits inherent to this heating mode have been reached in regard to speed as well as to hardness and restriction of the transition area.
There has been proposed already in GB 2 139 614 to use a device for cutting glass under thermal shock by means of a laser beam in which the fraction of light energy transmitted through the glass plate is reflected by a mirror against this glass plate. A fraction of the reflected energy is again absorbed by this glass plate and another fraction is transmitted therethrough. This residual energy fraction is then returned against the laser which situation affects its operation.
There has also been proposed in EP-O 174 915 to use an apparatus for finishing metal parts which move along a trajectory by means of a laser beam, this apparatus comprising a mirror arranged on the internal surface of a hemi-ellipsoidal body portion. This body portion is provided with an opening for transmitting the laser beam aimed at one of the foci of the ellipsoid located along the path of displacement of the metal parts, this being at a skew angle to prevent the reflected portion of light energy from being returned against the laser. In this embodiment, the energy condensed at the first focus is reflected by the mirror toward the second focus but, thereafter, the residual beam reflected again by the ellipsoidal mirror is sent back through the entrance opening.
Although this second embodiment avoids the drawbacks of the foregoing first embodiment, it does not however permit capturing the whole laser beam energy which finally exits through the entrance at a different angle, nor does it make it possible to return the reflected portions of this beam to a same given point of the path followed by the metallic parts; after each reflection, the light energy is driven against a different point of this path.
Yet, in order that the hardening be uniform, well delineated and substantially without a hardness gradient, for instance in the case of teeth cut from the edge of a wire, it is necessary, in view of the high density of energy supplied, to simultaneously heat the whole surface of the tooth for a very short time. This heating mode ensures that the heat is evenly distributed and makes it also possible to decrease the irradiation time because the efficiency is increased and to also avoid burning the edges of the teeth.